Notmuch is an answer to Sup. Sup is a very good email program written by
William Morgan (and others) and is the direct inspiration for Notmuch.
Notmuch began as an effort to rewrite performance-critical pieces of Sup
in C rather than ruby. From there, it grew into a separate project. One
significant contribution Notmuch makes compared to Sup is the separation
of the indexer/searcher from the user interface.

Notmuch provides a library interface so that its
indexing/searching/tagging features can be integrated into any email
program. Notmuch integrates very well with the mutt client for very fast
searching in all mails.
